The system of the invention is thus a member of the family of power line carrier transmission systems, and it may be applied, in particular, to concentrating and remotely reading electricity meter data, to remotely controlling a set of circuit breakers, to transmitting commands or information for automated homes, and more generally to automating an electricity power grid and the set of loads or members that are connected thereto.
Moreover, the invention is equally applicable to low tension networks, i.e. networks in which the voltage typically lies in the range 100 volts to 400 volts, and to medium tension networks, i.e. networks in which the tension typically lies in the range 3 kilovolts to 35 kilovolts, with the common characteristic being that the channel used for transmitting data is under all circumstances constituted by a line of a network or grid for distributing electricity.
The problems encountered in developing such carrier systems are numerous.
Firstly, transmission at relatively low frequencies, and in particular below 9 kHz, conflicts with the presence of harmonics of the fundamental frequency of the grid (generally 50 Hz or 60 Hz), which make it very difficult to detect the signals being transmitted.
In practice, transmission and low frequencies, and a fortiori, at very low frequencies, requires high power injectors to be used, and is thus necessarily expensive and very bulky.
Although transmission at high frequency makes it possible to escape from interference from the harmonics of the grid frequency and to reduce the size of the injectors considerably, it is impeded, in contrast, by other difficulties, namely both the possibly significant attenuation of the transmitted signal which depends on the number and the nature of the apparatus simultaneously connected to the electricity line, and the presence of impulse noise on the electricity line within the frequency band used for transmission.
For example, in most conventional systems, such impulse noise may, at least at some instants, cause the signal-to-noise ratio to pass below the minimum value for detecting the signal, which minimum value is generally around 5 dB.
Although it is known for all of these reasons to use spread-spectrum systems for transmitting data over electricity grids, and although such systems are perfectly satisfactory in some applications, all of them nevertheless present the drawback of being generally relatively complex and of permanently distributing energy over a very wide frequency band, and depending on circumstances, a large fraction of the very wide energy band will only be capable of providing a signal-to-noise ratio that is much too low to provide useful assistance in achieving successful transmission.
A system is known from U.S. Pat. No. 4,479,215 that complies with the preamble of the present description, i.e. that constitutes a carrier system using frequency modulation over a relatively narrow band selected from a set of predetermined frequency bands.
Although it is then possible, in theory, to concentrate all of the transmission energy in a band that is useful for transmission purposes, the use of such modulation itself gives rise to a new problem, namely that of selecting the band and of ensuring that both the transmitter and the receiver make use of it in coordinated manner.
In the prior art, this problem is resolved both in the context of carrier transmission over electricity lines, as described in U.S. Pat. No. 4,479,215, itself, and in the context of radio transmission (which is nevertheless marked by difficulties that are fundamentally different), as described in German patent DE 2,039,409, by basing coordination of the transmitter and of the receiver on a priori knowledge by both of them of the order in which the various frequency bands will be tried.
For example, according to U.S. Pat. No. 4,479,215, any new transmission is made on a predefined frequency band known to the transmitter and to the receiver, and if the transmitter fails to receive an acknowledgement from the receiver, it concludes that transmission has not worked; under which circumstances it sends a frequency skip instruction to all potential receivers thereof, requesting said receivers to monitor the arrival of a message on a second frequency band, likewise predefined and known in advance by the transmitter and by the receivers.
Although particular means are provided in U.S. Pat. No. 4,479,215 for increasing the probability that the receivers do indeed detect the frequency skip instruction, and are thus, in theory, informed that the band to be monitored has changed, three problems nevertheless remain associated with this mode of operation: firstly, the probability remains relatively high that a receiver may not detect the frequency skip signal under all transmission conditions; in addition, the probability of a receiver detecting an interference signal and interpreting it as a frequency skip signal is even higher; and finally and above all, the probability of a receiver detecting a message is exactly zero in each of the above two cases since the receiver is then necessarily set to a frequency band that is different from the band over which the message is being transmitted.
In this context, a specific object of the invention is to provide a line carrier communication system capable of adapting itself to data transmission over a plurality of frequency bands, but in which the adapting process does not rely on a priori knowledge by the transmitter and the receiver of the order in which the various frequency bands are to be tried, thereby avoiding the above-mentioned faults.